The Pentagon has decided to resume flights on its F-35 fighter jets, after the whole fleet was grounded last week.

A cracked turbine blade found on a plane prompted the suspension. But tests showed that this was a “unique” problem and not a design flaw, engine maker Pratt and Whitney said.

Thousands of F-35s are due to be made for the US and its foreign partners.

The F-35 is the Pentagon’s most expensive weapons programme, with a cost of nearly $400 billion.

The fault was detected during a routine inspection of an air force version of the jet (F-35A) at Edwards Air Force Base in California.

But on Thursday a spokesman for Pratt and Whitney, Matthew Bates, told Reuters news agency: “The team has determined that root cause is sufficiently understood for the F-35 to safely resume flights.”

Extensive tests on the plane’s engine revealed the crack was a result of the “unique operating environment” of the test flight, and was not a widespread issue, he added.

The engine had been running at high temperatures for four times longer than a normal F-35 flight, causing a separation of the “grain boundary” on one blade, Matthew Bates explained.

The Pentagon later confirmed that all its 51 planes had been cleared to resume flights.

The Pentagon has decided to resume flights on its F-35 fighter jets, after the whole fleet was grounded last week

Last week’s order to ground the planes – in the US air force, army and Marine Corps – marked the second time in two months planes from the F-35 range have been grounded.

The Marine Corps variant (F-35B), a short take-off and vertical landing variant (STOVL), was grounded for nearly a month after a manufacturing defect caused a fuel line to detach just before a training flight in January.

The F-35 programme includes partners from nine countries.

The construction of the plane has been plagued by problems – it is seven years behind schedule and has required numerous re-designs because of delays in software delivery and bulkhead cracks.

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